In a skillet cook bacon over medium-high heat until bacon is crisp. Transfer bacon to slow cooker. Add chicken, salt, and pepper to skillet and cook until browned. Transfer chicken to slow cooker with bacon.
Add mushrooms and cook 5 minutes, transfer to slow cooker with meat. Add carrots, onions, and garlic and cook until softened. Add vinegar to deglaze and cook until reduced about 2 minutes. Add to slow cooker with other ingredients.
Add broth, mustard, bay leaves, and milk. Cook on LOW for 4 hours.
With 30 minutes before serving, melt butter in a saucepan over medium-high heat until foamy. Add flour, cook, stirring constantly, 1-2 minutes until starting to smell toasty. Using a ladle add 1 cup of the slow cooker broth to the roux and stir to combine, bringing up to a simmer. Pour mixture into slow cooker and cook another 30 minutes to allow the stew to thicken.
Eat as is or serve over your choice of rice! Sprinkle fresh parsley over the top of the stew.